The present invention relates to a discrete multi-tone (DMT) modem. More particularly, the present invention relates to a method and apparatus for time-domain equalization in a frequency division multiplexing (FDM) based DMT modem to improve echo suppression, and reduce the effects of leakage of out of band frequency noise into in band signals.
Much interest has been expressed recently in DMT modems to increase bandwidth with various communication schemes, especially those digital subscriber line schemes commonly referred to as xDSL systems, such as ADSL. For example, asymmetric digital subscriber line (ADSL) was conceived originally for video-on-demand type applications, but the focus is now on providing higher speed Internet services, such as the World Wide Web. The asymmetry in ADSL refers to the allocation of available bandwidth and means that it is faster (i.e.xe2x80x94has more allocated bandwidth) in the downstream (towards the subscriber) direction and slower in the upstream (towards a central office) direction. Some applications, such as browsing on the Internet, do not generally demand symmetric data rates and can take advantage of an asymmetric system.
ADSL converts existing twisted-pair copper telephone lines into access paths for multimedia and high-speed data communications. ADSL can transmit more than 6 megabits per sec (Mbps) (optionally up to 8 Mbps) to a downstream subscriber from a central office, and as much as 640 kilobits per second (kbps) (optionally up to 1 Mbps) upstream from a subscriber to the central office. Such rates expand existing access modem capacities by a factor of 50, or more, without new cabling.
ADSL was designed for residential or small-office, home-office type services and thus, it was designed from the outset to operate with the analog voice signals of Plain Old Telephone Service (POTS) simultaneously on the same line, such that an additional copper line is not needed. Generally, the POTS channel is split off from the digital modem by filters to provide uninterrupted POTS, even if the ADSL circuit fails.
Unlike previous copper line technologies, an ADSL system does not need manual pre-adjustment to accommodate line conditions. Instead, the ADSL modem automatically analyzes the line, as part of the process of establishing a connection, and adapts itself to start up the connection. This adaptation process can continue, once the connection is started, as the modem compensates for ongoing changes, such as those due to temperature or other environmental factors. Factors that can affect ADSL transmission include the data rate, the gauge thickness of the copper cable, the distance between the central office and the subscriber and the amount of interference present on the line.
To support bi-directional channels, ADSL modems allocate the available bandwidth by FDM, where non-overlapping bands are assigned for the downstream and upstream data. DMT, which has now been accepted by ANSI as the standard line code for ADSL transmission, divides an input data stream among several sub-channels, each sub-channel having the same amount of bandwidth but at different center frequencies. Sub-channels can have different bit rates, as discussed below. Using many sub-channels with very narrow bandwidths means the theoretical channel capacity, as calculated according to Shannon""s law, can be approached. Generally, DMT was chosen because it is particularly well suited for transmission over copper line at the operating frequency bands. DMT also copes well with the typical noise and impulses that exist in the residential (subscriber) twisted-wire pair environment.
The sub-channels into which a channel is divided, commonly referred to as tones, are quadrature amplitude modulation (QAM) modulated on a separate carrier, commonly called a subcarrier, and the subcarrier frequencies are multiples of one basic frequency. The ANSI standard ADSL system has a theoretical maximum of 256 frequency sub-channels for the downstream data and 32 sub-channels for the upstream, though, in reality, line conditions, interference and other considerations reduce the actual available number of sub-channels. The frequency difference between two successive sub-channels is 4.3125 Khz. In a DSL-Lite or G.Lite system, the number of downstream data streams is halved, eliminating those at the higher frequencies.
As mentioned above, data to be transmitted is QAM modulated so that each sub-channel can transmit multiple bits and bit rates can vary between sub-channels. As the subscriber loops between the central office and a subscriber generally exhibit variations in gain and phase with frequency, each sub-channel can be arranged to carry a different number of bits appropriate for its frequency on the particular subscriber line. By assigning different numbers of bits to different sub-channels, each sub-channel can operate at an optimal, or near optimal, bit rate for the bandwidth available in the subscriber loop. Sub-channels at frequencies where the signal-to-noise ratio is low can have lower numbers of bits assigned to them, while sub-channels at frequencies with higher signal-to-noise ratios can have higher numbers of bits assigned to them, to keep the probability of a bit error constant across the subcarriers.
DMT modems are subject to a variety of noise sources that can cause undesirable interference. Gaussian, or white, noise is always present. Cross talks due to services on the loop, such as POTS or integrated services digital network (ISDN), or on other adjacent lines, can cause interference, especially in the upstream band. Echo due to the duplex nature of transmission in the DMT modem can also be a significant source of interference. Particularly in G.Lite DMT modems, an often unrecognized source of noise is caused by xe2x80x9cout of bandxe2x80x9d interference, such as radio frequency interference (RFI) which can be converted into xe2x80x9cin bandxe2x80x9d interference by the processing inherent to the receiver.
DMT modems contain advanced digital signal processing (DSP) systems to perform time domain to frequency domain conversions and to model the distortions caused by the noise and produce automatic corrections. Such models are implemented by various digital filters and, typically, the DMT modem includes filters to reduce echo and equalize frequency and time domain response, as well as various band pass filters. The performance of the DMT modem, particularly the usable bandwidth and achievable bit rate, is determined in large part by efficiency of these filters. However, there is always a trade-off between the optimum achievable performance and the relative complexity and cost, both in terms of components and processing time, of the digital filters.
It is therefore desired to have a DMT modem that can more efficiently model and implement digital filtering, particularly for the suppression of echo and out of band interference.
It is an object of the present invention to provide a novel method and apparatus for time domain equalization to suppress at least a portion of the line echo, and leakage from out of band frequencies in an FDM-based, DMT modem.
According to a first aspect of the present invention, there is provided a method for suppressing echo and out of band interference in a discrete multi-tone modem having a time domain equalizer, comprising the steps of:
(i) determining an estimated channel impulse response;
(ii) determining an estimated out of band noise power spectral density;
(iii) performing a statistical analysis of said estimated channel impulse response and said estimated out of band noise power spectral density;
(iv) applying a constraint to said analysis to determine a target impulse response; and
(v) determining parameters for said time domain equalizer from said target impulse response.
In a further aspect, the present invention provides a discrete multi-tone modem comprising:
a channel impulse response estimator for estimating a channel impulse response;
an out of band noise power spectral density estimator for estimating an out of band noise power spectral density;
calculating means for performing a statistical analysis of said channel impulse response and said out of band noise power spectral density, constraining said analysis to determine a target impulse response; and determining time domain equalization parameters from said target impulse response; and
a time domain equalizer having taps set to said determined parameters.
In yet a further aspect of the present invention, there is provided a time domain equalizer for a discrete multi-tone modem, comprising:
a channel impulse response estimator for estimating a channel impulse response;
an out of band noise power spectral density estimator for estimating an out of band noise power spectral density;
calculating means for performing a statistical analysis of said channel impulse response and said out of band noise power spectral density, constraining said analysis to determine a target impulse response; and determining time domain equalization parameters from said target impulse response; and
at least one digital filter set to said determined parameters.